The object of this invention is a process for storing heat in the form of latent heat which comprises storing the heat originating from a hot fluid by passing said fluid into a thermally insulated storage element. This storage element contains a substance capable of fusion/solidification under the influence of the introduction or abstraction of heat by means of a sweeping fluid.
It is known to store heat produced from different sources of energy in the form either of perceptible heat of a fluid, generally water, or of a solid such as gravel, or in the form of latent heat of phase change of suitable body at the level of temperature used.
The first type of storage requires devices of substantial volume, the heat capacity of the substances used being in the proximity of 0.5 cal.g.sup.-1. (.degree.C.).sup.-1, that of the water reaching the exceptional value of 1 cal.g.sup.-1. (.degree.C.).sup.-1. The temperature level of abstraction of the stored heat is clearly less than that of the introduction of the heat by the source of energy. In the case of solids such as gravel there is a great resistance to thermal transfer between the thermo-conductive fluid and the medium storing the heat which also impairs interest in the process.
However, the storage in the form of latent heat of fusion makes it possible to store much more heat per unit of volume of storage and to minimize the loss of temperature level between the stored heat and the abstracted heat. Storage in the form of latent heat of fusion has been extensively studied for a very broad temperature range, making use of different substances, mainly fusible salts, paraffins, polymers and metal alloys.
However, practical application of the storage of heat in the form of latent heat is difficult. If a receptacle traversed by a coil through which the thermo-conductive fluid circulates is filled with a substance susceptible to melting or solidifying at a temperature compatible with the introduction and abstraction of heat required, the flow of heat on the walls of the coil would be very small during the solidification phase when heat is being withdrawn. The transfer of heat takes place only by conduction through an increasing thickness of solid. In the fusion phase corresponding to heat storage, the transfer of heat takes place only by natural conduction and convection. The process is not efficient due to the generally high viscosity of substances in the proximity of their solidification point.
In order to reduce said inconveniences and increase to the maximum the thermal flow between the medium storing the heat and the thermo-conductive fluid, it is necessary to increase the exchange surface and reduce the thickness of the storing medium.
Different solutions have been proposed. One of the best solutions is to encapsulate the fusible substance in the form of a granular medium: balls or spheroids in general. However, unless the capsules are of small diameter, which poses a problem of microencapsulation, and use at relatively low temperatures - since the substances that are used for encapsulation are generally polymers - the solution is of no great interest.